1. Field of the Invention
The present invention relates to a secondary air controlling apparatus to be used to clean the exhaust gases of internal combustion engines for vehicles.
2. Description of the Prior Art
As a way of cleaning the exhaust gases of engines for vehicles, there is known an air injection system wherein secondary air is fed into the exhaust passsage to burn and decompose the unburnt harmful component in the exhaust gases. In this cleaning system, if there is a large amount of the unburnt harmful component, when the secondary air is fed with sufficient oxygen, an extreme after-burning will occur. Such after-burning is liable to melt and frequently damage the cleaning system and attachments thereto, such as a catalyst holding member.
Particularly, during deceleration, since the opening angle of the throttle valve of the carburetor is small, less air-fuel mixture is sucked in. Also, the pressure in the intake pipe is lowered, and the liquid fuel on the intake pipe wall is gasified. Therefore, a rich mixture is sucked into the cylinder, and the unburnt harmful component in the exhaust gases increases. This condition results in the after-burning which is undesirable to the cleaning system as described above. Under such conditions, if the secondary air is interrupted, the after-burning can be prevented with the feeding of the rich mixture. Consequently, the unburnt harmful component will be discharged out of the exhaust system without cleaning.
Therefore, during deceleration, it is desirable, on the one hand, to interrupt the feeding of secondary air into the exhaust system to prevent the extreme after-burning and on the other hand, to introduce secondary air into the intake passage to dilute the rich mixture in the intake pipe and improve the combustion within the combustion chamber. The feeding of secondary air into the intake pipe must be made only for an initial period of the deceleration, because if secondary air is continuously provided, the mixture will become too lean. The feeding of secondary air into the exhaust system should be resumed after the initial period. However, this system requires many valves and complicated structure to make the respective operations, and is not adapted for practical use. This problem is solved by the present invention.
Further, when this kind of cleaning system is provided, just after the ignition key is switched off, the engine still rotates for a little while due to its momentum, and the air-fuel mixture sucked into the combustion chamber under such condition is not burnt, but rather is discharged as it is into the exhaust passage. If secondary air is fed into the exhaust passage through a reed valve with a negative pressure part of exhaust pulsation, the unburnt harmful component will combine with sufficient oxygen and explosively burn in the cleaning system remaining at a high temperature, an extreme after-burning will occur, and the exhaust cleaning and exhaust system will be damaged. Therefore, when the engine is turned off, it is also desirable to interrupt the feeding of secondary air into the exhaust passage.
As a way of preventing the after-burning just after the ignition key is switched off, there is a known method wherein, just after the ignition key is turned off, secondary air is introduced downstream of the throttle valve to suppress the rise of the negative suction pressure, to regulate the fuel sucked in from the carburetor, and to prevent the unburnt mixture from being discharged into the exhaust passage. However, in this method, the intake passage always communicates with the atmosphere through the secondary air introducing passage when the engine is off; the liquid fuel on the inner wall of the intake passage evaporates and leaks out to the atmosphere. This is undesirable from the viewpoint of air pollution, and is another problem solved by the present invention.
The present invention solves the above-mentioned various problems in systems for cleaning exhaust gases with secondary air.